Apparatus for practicing the vacuum-assisted, countergravity casting process using a gas permeable mold is described in such prior art patents as the Chandley et al U.S. Pat. No. 3,900,064 issued Aug. 19, 1975. U.S. Pat. No. 4,340,108 issued July 20, 1982 and U.S. Pat. No. 4,606,396 issued Aug. 19, 1986.
Typically, the vacuum-assisted, countergravity casting apparatus includes a gas permeable mold, a vacuum chamber disposed about the mold and means for immersing a lower portion of the mold in an underlying pool of the melt while evacuating the vacuum chamber to draw the melt upwardly from the pool into one or more mold cavities of the mold.
In practicing the vacuum-assisted, countergravity casting process to cast a plurality of separate, unconnected parts (i.e., castings), the gas permeable mold comprises a central fill sprue (also referred to as a riser) having an open lower end adapted for immersion in an underlying pool of melt and a plurality of mold cavities each connected in melt flow relation to the fill sprue via a laterally extending ingate therebetween. During casting, the melt is drawn upwardly from the underlying pool through the fill sprue and into the mold cavities via the lateral ingates. Once the mold cavities are filled with the melt, the vacuum chamber/mold are raised to withdraw the sprue open lower end from the pool and the vacuum established in the vacuum chamber is released (i.e., ambient pressure is provided the vacuum chamber) to cause the melt in the fill sprue to drain by gravity back into the underlying pool. Drainage of the fill sprue in this manner improves the overall economies of the vacuum-assisted, countergravity process in that the overall casting cycle time is reduced, a plurality of separate castings unconnected to a central metal sprue are produced (eliminating the need to separate the castings from one another) and less melt is used to produce the castings.
However, in draining the melt from the mold fill sprue in the manner described, siphoning surges are created in the mold cavities as the melt (especially a heavy melt such as molten iron) drains from the sprue open lower end. These siphoning surges are harmful in that some of the melt filling the mold cavities can be siphoned out of the mold cavities and result in the production of defective castings.
Although the Chandley U.S. Pat. No. 4,112,997 issued Sept. 12, 1978 describes a vacuum-assisted, countergravity apparatus and method wherein the melt is drained from the mold fill sprue without siphoning of melt from the mold cavities, the patent requires placement of an apertured stabilizing screen in the ingate between each mold cavity and the fill sprue. The stablizing screens promote early solidification of the melt thereon after the mold cavities are filled to prevent melt drainage from the mold cavities when the vacuum is subsequently released to drain the melt from the fill sprue. However, incorporation of such stabilizing screens into each mold ingate significantly increases the complexity and cost of the gas permeable casting mold and thus adversely affects the economies of the vacuum-assisted, countergravity casting process.
It is an object of the present invention to provide an improved vacuum-assisted, countergravity casting apparatus and method that enable drainage of a mold fill sprue without siphoning melt from melt-filled mold cavities and that eliminate the need for incorporation of stabilizing screens or other additional components into the casting mold.
It is another object of the present invention to provide an improved apparatus and method for the vacuum-assisted, countergravity casting of a melt wherein a positive differential pressure is so established between a mold fill sprue and one or more melt-filled mold cavities after the mold cavities are filled with the melt as to cause the melt in the mold fill sprue to drain therefrom for return to an underlying source without siphoning of the melt from the melt-filled mold cavities.
It is another object of the present invention to provide an improved apparatus and method for the vacuum-assisted, countergravity casting of a melt wherein pressurized gas is introduced into a mold fill sprue as the mold is communicated to an underlying source of the melt for discharge from the sprue toward the melt in a manner to blow impurities and debris floating on the melt away from the melt surface region where the mold is immersed, thereby reducing entry of impurities/debris into the mold.